Cathode active material for lithium secondary battery

ABSTRACT

Disclosed herein is a cathode active material based on lithium nickel oxide represented by Formula 1, wherein the lithium nickel oxide has a nickel content of at least 40% among overall transition metals and is coated with a polymer having a melting point of 80 to 300° C. at a surface thereof. A lithium secondary battery having the disclosed cathode active material has advantages of not deteriorating electrical conductivity while maintaining high temperature stability, so as to efficiently provide high charge capacity.

TECHNICAL FIELD

The present invention relates to a cathode active material for lithiumsecondary batteries and, more particularly, to a cathode active materialbased on lithium nickel oxide represented by Formula 1, wherein thelithium nickel oxide has a nickel content of at least 40% among overalltransition metals and is coated with a polymer having a melting point of80 to 300° C.

BACKGROUND ART

With technological advancement and demand for mobile instruments, demandfor secondary batteries as an energy source is rapidly increasing. Amongsuch secondary batteries, a lithium secondary battery having high energydensity and working potential, a long life cycle, and reducedself-discharge is widely used in the related art.

As to cathode active materials for a lithium secondary battery, lithiumcontaining cobalt oxide (LiCoO₂) is widely used. Additionally, lithiumcontaining manganese oxides such as LiMnO₂ with a lamellar crystalstructure, LiMn₂O₄ with a spinel crystal structure, etc., and lithiumcontaining nickel oxide (LiNiO₂) may also be used.

Among such cathode active materials, although LiCoO₂ with excellentphysical properties such as cycle properties is widely used, thismaterial encounters disadvantages including, for example, low safety,high cost due to scarcity of cobalt as a natural resource, limitation inlarge-scale use as a power source in electric vehicle applications, andthe like.

Lithium manganese oxides such as LiMnO₂, LiMn₂O₄, etc., comprisemanganese, which is abundant and environmentally beneficial so as toreplace LiCoO₂, thus attracting considerable attention. However, suchlithium manganese oxide has drawbacks such as a low charge capacity andpoor cycle properties.

Meanwhile, lithium nickel based oxide such as LiNiO₂ has economicmerits, compared to the cobalt oxide and, when charged at 4.3V, exhibitshigh discharge capacity. A reverse capacity of the doped LiNiO₂ is about200 mAh/g which is more than that of LiCoO₂ (about 165 mAh/g).Accordingly, even though average discharge potential and volumetricdensity are somewhat small, a commercially available battery containinga cathode active material shows improved energy density. Under suchcircumstances, in order to develop a high capacity battery, studies andinvestigations into nickel based cathode active materials are activelybeing conducted. However, practical application of LiNiO₂ cathode activematerials is substantially restricted owing to the following problems.

First, LiNiO₂ oxide exhibits rapid phase transition in a crystalstructure due to change of volume involved in a charge-discharge cycle,in turn causing particle fracture and generating pores in a grainboundary. Therefore, absorption and discharge of lithium ions areprevented and polarization resistance is increased, thus deterioratingcharge-discharge performance. In order to solve these problems,according to a conventional process, Li source is excessively used andreacts in an oxygen atmosphere to produce LiNiO₂ oxide. The producedcathode active material has drawbacks in that a structure is expandedand unstable due to atomic repulsion of oxygen atoms during charge of abattery and cycle properties are seriously deteriorated by repeatedcharge-discharge.

Second, LiNiO₂ encounters a problem of excessive gas generation duringstorage or charge-discharge cycle. This is because heat treatment isperformed while excessively adding Li source to form an excellentcrystal structure during production of LiNiO₂, and therefore, awater-soluble base such as Li₂CO₃, LiOH, etc. as a reaction residueremains between primary particles and is degraded or reacts with anelectrolyte, in turn generating CO₂ gas during charge. Furthermore,since a LiNiO₂ particle substantially has a secondary particle structureformed by aggregation of primary particles, an area in contact with theelectrolyte is increased and the foregoing problem becomes more serious,thus causing swelling of the battery and decreasing high temperaturestability.

Third, when LiNiO₂ is exposed to air and/or moisture,chemical-resistance is drastically decreased at a surface of the oxideand, due to high pH, an NMP-PVDF slurry begins to be polymerized, inturn causing gellation thereof. The foregoing characteristics may causeserious processing problems in the manufacture of batteries.

Fourth, high quality LiNiO₂ cannot be prepared by simple solid-phasereaction, unlike the LiCoO₂ production method. Any LiNiMO₂ cathodeactive material comprising Co as a necessary dopant, and other dopantssuch as Mn, Al, etc. is substantially produced by reacting a lithiummaterial such as LiOH.H₂O with a composite transition metal hydroxideunder an oxygen atmosphere or a synthetic gas atmosphere (that is, aCO₂-free atmosphere), thus requiring high production costs. If anyadditional process such as washing or coating is conducted in order toremove impurities during production of LiNiO₂, production costs are dulyincreased. Accordingly, conventional technologies have focused ingeneral on improvement of the LiNiO₂ production process as well ascharacteristics of LiNiO₂ cathode active material.

A lithium transition metal oxide wherein nickel is partially substitutedwith other transition metals such as manganese, cobalt, etc. has beenproposed. This oxide is a metal-substituted nickel based lithiumtransition metal oxide with excellent cycle properties and capacity.However, in the case of using the oxide for a long time, cycleproperties are drastically deteriorated and other problems such asswelling caused by gas generation in a battery, reduced chemicalstability, and so forth, were not sufficiently overcome.

The cause of the foregoing facts is believed to be that: the nickelbased lithium transition metal oxide is in a secondary particle formobtained by aggregation of small primary particles; therefore, lithiumions are transported toward a surface of an active material and reactwith moisture or CO₂ in the air to generate impurities such as Li₂CO₃,LiOH, etc.; impurities generated by residues remained after productionof nickel based lithium transition metal oxide may decrease cellcapacity; or the impurities are decomposed inside the battery togenerate gas, in turn causing swelling of the battery.

Accordingly, there is still a requirement for development of noveltechniques to solve high temperature stability problems due toimpurities while utilizing a lithium nickel based cathode activematerial suitable for increasing capacity of a battery.

DISCLOSURE Technical Problem

Therefore, the present invention is directed to solving conventionalproblems described above and to overcoming technical restrictions inrelated arts.

As a result of extensive studies and a number of experiments executed bythe present inventors, it was found that a cathode active materialprepared by applying a polymer having a melting point of 80 to 300° C.to a surface of a lithium nickel oxide may achieve improved hightemperature stability and, thereby, the present invention wassuccessfully completed.

Technical Solution

Accordingly, the present invention provides a cathode active materialincluding a lithium nickel oxide represented by Formula (1), wherein thelithium nickel oxide has a nickel content of at least 40% among overalltransition metals and is coated with a polymer having a melting point of80 to 300° C.

Li_(x)Ni_(y)M_(1-y)O₂  (1)

wherein 0.95≦x≦1.15, 0.4≦y≦0.9, and M is at least one selected from agroup consisting of stable elements at six-coordination such as Mn, Co,Mg, Al, etc.

The cathode active material of the present invention has advantages ofproviding high capacity and blocking a path of ions and electrons at anabnormally high temperature thanks to application of a polymer having amelting point of 80 to 300° C. to a surface of the active material,thereby noticeably improving high temperature stability.

As to a lithium transition metal oxide, when an unstable surface thereofis exposed to an electrolyte and subjected to internal and/or externalimpact, oxygen is discharged in turn causing rapid exothermic reaction.Such exothermic reaction may be accelerated by an electrolyte, anelectrolyte salt, etc. other than a cathode active material.

However, the cathode active material of the present invention coatedwith the polymer having a melting point of 80 to 300° C. on a surface oflithium nickel oxide, forms a blocking interface between an electrolyteand the cathode active material so that reactivity of the activematerial with the electrolyte is considerably decreased, and is fused ata high temperature of more than 80° C. and increases internal resistanceat an abnormally high temperature of a battery so as to prevent mobilityof ions and electrons, thus inhibiting ignition and/or explosion of thebattery and enhancing high temperature stability. Hereinafter, thepresent invention will be described in detail.

The cathode active material of the present invention is formed byapplying a polymer having a melting point of 80 to 300° C. to a surfaceof lithium nickel oxide.

That is, the lithium nickel oxide is a cathode active material with highNi content of not less than 40% among overall transition metals. Assuch, if the active material has higher Ni content than other transitionmetals, a fractional ratio of divalent nickel is relatively high. Inthis case, since an amount of charge to transport lithium ions isincreased, high charge capacity is provided.

Constitutional composition of the foregoing lithiumnickel-manganese-cobalt oxide must satisfy a specific condition definedby Formula 1.

That is, lithium (Li) content ‘x’ ranges from 0.95 to 1.15 and, if Licontent exceeds 1.5, safety may be decreased during cycling due to highvoltage (U=4.35V) at a particular temperature of 60° C. In contrast, ifx<0.95, rate properties and reverse capacity are reduced.

Alternatively, Ni content ranges from 0.4 to 0.9, which is relativelyhigher than those of manganese and cobalt. If Ni content is less than0.4, the cathode active material cannot have high capacity. On thecontrary, when the nickel content is above 0.9, safety is drasticallydecreased.

M refers to at least one selected from stable elements atsix-coordination such as Mn, Co, Mg, Al, etc. Preferably, M is Mn or Co.

A preferred example of the lithium nickel oxide represented by Formula 1may be one represented by Formula 1a below:

Li_(x)Ni_(y)Mn_(c)CO_(d)O₂  (1a)

wherein c+d=1−y, provided 0.05≦c≦0.4 and 0.1≦d≦0.4.

When Mn content ‘c’ is less than 0.05, safety is deteriorated. Withc>0.4, an amount of charge to transportions is reduced, thus decreasingthe charge capacity.

In addition, Co content ‘d’ ranges from 0.1 to 0.4. If d>0.4, that is,the cobalt content is excessively high, raw material costs are generallyincreased while Co⁴⁺ is unstable during cell charging, thus decreasingsafety of the battery. On the other hand, if d<0, that is, the cobaltcontent is too low, it is difficult to simultaneously achieve desirablerate properties and high power density of a battery.

Increase in particle size of the lithium nickel oxide may improvestability of crystal particles, enabling easy manufacture of a batterycontaining the same and improving efficiency of a manufacturing process.However, if the particles are too large, a surface area on which theactive material reacts with an electrolyte contained in a battery cellis reduced, causing drastic deterioration in characteristics such ashigh voltage storage, rate properties, etc. On the contrary, if aparticle size of the lithium nickel oxide is excessively decreased,structural stability such as high temperature characteristics isdeteriorated. Considering these problems, the lithium nickel oxide mayhave an average particle diameter (D50) of 3 to 20 μm. Briefly, suchstructural stability including high temperature characteristics may berelatively favorable while reducing adverse effects such as electrolytedegradation.

Preferably, the lithium nickel oxide may comprise secondary particlesformed by agglomeration of primary particles. The primary particle hasan average particle diameter of 0.01 to 8 μm, while an average particlediameter of the secondary particle preferably ranges from 3 to 20 μm.

As a size of the primary particle is decreased, excellent rateproperties may be embodied. However, if the particle diameter is toosmall, a specific surface area of the primary particle is considerablylarge. Thereby, an amount of impurities present on a surface of thelithium nickel oxide is increased and a structure of the secondaryparticle formed by agglomeration of primary particles may be broken dueto pressure applied during manufacture of a cathode. On the other hand,when the particle diameter of the primary particle is large, the amountof impurities is reduced and the structure of the secondary particle maybe preferably maintained. However, excessively large particle diametermay entail deterioration in rate properties.

Meanwhile, as a size of the secondary particle is decreased, mobility oflithium ions may be improved, in turn embodying excellent rateproperties. However, if the particle size is too small duringmanufacture of a cathode, some problems including, for example, decreasein dispersibility caused by particle aggregation, increase in amount ofbinder, reduction of cell capacity, and the like have been encountered.

With respect to lithium nickel oxide with high Ni content comprising thecathode active material of the present invention, as a content of Ni²⁺ions is increased during calcination, desorption of oxygen becomesserious at a high temperature. As a result, several problems, namely,decrease in stability of a crystal structure, widening of specificsurface area, increased impurity content, in turn increasing reactivityof the foregoing oxide with an electrolyte, and reduced high temperaturestability, etc. have been encountered.

Accordingly, the present invention provides a cathode active materialcomprising lithium nickel-cobalt-manganese oxide coated with a polymerhaving a melting point of 80 to 300° C., in order to improve hightemperature stability. If the melting point is less than 80° C.,internal resistance may be increased even under normal operationconditions, thus decreasing cell characteristics. When the melting pointis more than 300° C., it is difficult to obtain desired high temperaturestability.

That is, the polymer is fused at an abnormally high temperature of abattery and scorched and stuck on the surface of cathode active materialor flows into a gap of the cathode active material, thus blocking aconductive path and reducing mobility of ions and electrons. Therefore,by increasing internal resistance of a battery, progress ofelectrochemical reaction may be prevented, thereby inhibiting ignitionof the battery.

According to a preferred embodiment, the foregoing polymer may beinactivated by an electrolyte or an organic solvent. Since suchinactivated polymer is not removed during fabrication of an electrodeand/or elution of the polymer into an electrolyte or degradation thereofwhen the polymer is contained in a battery does not occur, therebybeneficially preventing decrease in cell performance.

The foregoing polymer may be at least one selected from a groupconsisting of polyethylene, polypropylene, polybutylene and polystyreneor a copolymer or blend comprising two or more thereof, although notduly limited thereto.

A molecular weight of the polymer is not particularly limited. However,if the molecular weight is too small, the electrolyte may be eluted out.In contrast, if the molecular weight is excessively large, viscosity ofa coating solution may be increased. Preferably, the polymer may havenumber-average molecular weight ranging from 1,000 to 1,000,000.

The above polymer may be applied to a surface of lithium transitionmetal oxide by chemical bonding or, otherwise, in consideration ofsimple processing and stability of lithium transition metal oxide, byphysical contact such as van-der-Waals force or electrostaticattraction, etc.

Such physical contact may be attained by any easy method using amechano-fusion device or Nobilta device for fusion. The mechano-fusiondevice utilizes physical rotation in a dried state to prepare a mixture,so as to form static coupling of constitutional components.

It is not necessary to completely coat lithium nickel-cobalt-manganeseoxide with the foregoing polymer, for the purpose of accomplishingfunctional effects of the present invention. Preferably, the polymer isuniformly applied to a part of an overall surface of the lithiumnickel-cobalt-manganese oxide to be coated. If coating area isexcessively large, mobility of lithium ions is reduced and rateproperties may be deteriorated. When the coating area is too small,desired effects may not be attained. Therefore, it is preferable to coatabout 20 to 80% of the overall surface of the lithiumnickel-cobalt-manganese oxide with the foregoing polymer.

Furthermore, coating thickness depends on various factors such as typesof organic solvent, an addition amount of polymer, an addition amount oflithium nickel-cobalt-manganese oxide, agitation speed, velocity, etc.,and therefore is not particularly limited. The coating thickness maypreferably range from 0.1 to 10 μm.

If a coating amount of the polymer is too small, coating effects may behardly attained. On the contrary, when the coating amount is too large,cell performance may be deteriorated. Therefore, the coating amount manrange from 0.5 to 10% by weight relative to a total weight of the activematerial.

The present invention also provides a lithium secondary batterycontaining the cathode active material described above. The lithiumsecondary battery may comprise, for example, a cathode, an anode, aseparator and a non-aqueous electrolyte containing lithium salt.

The cathode is fabricated by, for example, applying a mixture of thecathode active material, a conductive material and a binder to a cathodecollector and drying the coated collector. Optionally, a filler may beadded to the mixture. The anode is fabricated by applying an anodeactive material to an anode collector and drying the coated collectorand, if necessary, may further contain the foregoing ingredients.

The anode active material may include, for example: carbon and graphitematerials such as natural graphite, artificial graphite, expandablegraphite, carbon fiber, hard carbon, carbon black, carbon nanotubes,fullerene, activated carbon, etc.; metals alloyable with lithium such asAl, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt, Ti, etc. and compoundscontaining the same; composites of metals and compounds thereof withcarbon and graphite materials; lithium containing nitrides, and soforth. Among these, carbon based active materials, silicon based activematerials, tin based active materials, and/or silicon-carbon basedactive materials are more preferable and may be used alone or incombination of two or more thereof.

The separator is interposed between the cathode and the anode andconsists of a thin insulation film having high ion permeability andexcellent mechanical strength. A diameter of each pore in the separatorgenerally ranges from 0.01 to 10 μm and a thickness thereof generallyranges from 5 to 300 μm. Such separator may be fabricated using a sheetor non-woven fabric made of, for example, an olefin polymer such aspolypropylene having chemical resistance and hydrophobicity, glass fiberor polyethylene. When a solid electrolyte such as a polymer electrolyteis used, this electrolyte may also serve as the separator.

Another example of the separator may be an organic/inorganic compositeporous separator having an active film, characterized in that at leastone region selected from a polyolefin based separator substrate, asurface of the substrate and a part of a porous region in the activefilm is coated with a mixture of inorganic particles and a binderpolymer. Optionally, the inorganic particles may be applied to anelectrode side.

Such inorganic particle may include, for example, an inorganic particlewith a dielectric constant of 5 or more, an inorganic particleexhibiting piezo-electricity, an inorganic particle with lithium iontransfer ability, and the like.

The binder may include, for example: polyvinylidene fluoride; polyvinylalcohol; carboxymethyl cellulose (CMC); starch; hydroxypropyl cellulose;regenerated cellulose; polyvinyl pyrrolidone; tetrafluoroethylene;polyethylene; polypropylene; ethylene-propylene-diene terpolymer (EPDM);sulfonated EPDM; styrene-butylene rubber; fluorine rubber; differentcopolymers; high saponification polyvinyl alcohol, and the like.

The conductive material is used to improve conductivity of the electrodeactive material and may be added in an amount of 1 to 30 wt. % relativeto a total weight of an electrode mixture. The conductive material isnot particularly restricted so long as it exhibits conductivity whilenot causing chemical change of a battery. For example, the conductivematerial may comprise: graphite such as natural graphite or artificialgraphite; carbon black such as carbon black, acetylene black, ketchenblack, channel black, furnace black, lamp black, summer black, etc.; aconductive fiber such as carbon derivatives including carbon nanotubesor fullerenes, carbon fiber, metal fiber, etc.; metal powder such ascarbon fluoride, aluminum or nickel powder; a conductive whisker such aszinc oxide, potassium titanate, etc.; conductive metal oxide such astitanium oxide; a conductive material such as polyphenylene derivative,and the like.

A viscosity controlling agent refers to a component regulating viscosityof an electrode mixture in order to help processes for blending andapplying the electrode mixture to a collector to be more easilyperformed. The viscosity controlling agent is preferably added in anamount of up to 30 wt. % relative to a total weight of the electrodemixture. Examples of such viscosity controlling agent may includecarboxymethyl cellulose, polyvinylene fluoride, etc., although not dulylimited thereto. Optionally, the foregoing solvents may also serve as aviscosity controlling agent.

The filler used herein is an additional component to inhibit expansionof an electrode and is not particularly limited so long as it comprisesfibrous materials without causing chemical change of a battery. Forexample, the filler may be formed using olefin based polymer such aspolyethylene, polypropylene, etc. or a fibrous material such as glassfiber, carbon fiber, etc.

A coupling agent is another additional component to increase adhesionbetween an electrode active material and a binder, characterized inhaving at least two functional groups, and may be used in an amount ofup to 30 wt. % relative to a weight of the binder. An example of suchcoupling agent may be a material having at least two functional groupswherein one of the functional groups reacts with a hydroxyl or carboxylgroup present on a surface of silicon, tin or graphite based activematerial to form a chemical bond while another functional group reactswith a polymer binder to form another chemical bond. A preferred exampleof the coupling agents may be a silane based coupling agent including:triethoxysilylpropyl tetrasulfide; mercaptopropyl triethoxysilane;aminopropyl triethoxysilane; chloropropyl triethoxysilane; vinyltriethoxysilane; methacryloxypropyl triethoxysilane; glycidoxypropyltriethoxysilane; isocyanatopropyl triethoxysilane; cyanatopropyltriethoxysilane, etc., although not particularly limited thereto.

An adhesion promoter used herein is an additional component to improveadhesion of an active material to a collector and may be added in anamount of not more than 10 wt. % relative to the binder. Examples of theadhesion promoter may include oxalic acid, adipic acid, formic acid,acrylic acid derivatives, itaconic acid derivatives, and the like.

A molecular weight controller may include, for example,t-dodecylmercaptan, n-dodecylmercaptan, n-octylmercaptan, etc. Across-linking agent may include, for example, 1,3-butanediol diacrylate,1,3-butanediol dimethacrylate, 1,4-butanediol diacrylate, 1,4-butanedioldimethacrylate, aryl acrylate, aryl methacrylate, trimethylolpropanetriacrylate, tetraethyleneglycol diacrylate, tetraethyleneglycoldimethacrylate, divinylbenzene, and the like.

The collector of the electrode is a part in which electrons move duringelectrochemical reaction of the active material and, based on types ofthe electrode, may be classified into an anode collector and a cathodecollector.

The anode collector is generally fabricated with a thickness of 3 to 500μm. So long as the anode collector exhibits conductivity and does notcause chemical change of a battery, materials of the anode collector arenot particularly restricted and may include, for example, copper,stainless steel, aluminum, nickel, titanium, calcined carbon, copper orstainless steel surface-treated with carbon, nickel, titanium, silver,etc., or aluminum-cadmium alloy, and so forth.

The cathode collector is generally fabricated with a thickness of 3 to500 μm. So long as the cathode collector exhibits high conductivity anddoes not cause chemical change of a battery, materials of the cathodecollector are not particularly restricted and may include, for example,stainless steel, aluminum, nickel, titanium, calcined carbon, oraluminum or stainless steel surface-treated with carbon, nickel,titanium, silver, etc.

The collector may form fine unevenness on a surface thereof in order toreinforce binding of an electrode active material and be utilized indifferent forms such as a film, a sheet, a foil, a net, a porous body, afoam, a non-woven fabric, and the like.

The lithium containing non-aqueous electrolyte used herein may comprisea non-aqueous electrolyte and a lithium salt.

The foregoing non-aqueous electrolyte may be an aprotic solventincluding, for example: N-methyl-2-pyrrolidinone; propylene carbonate;ethylene carbonate; butylene carbonate; dimethyl carbonate; diethylcarbonate; γ-butyrolactone; 1,2-dimethoxyethane; tetrahydroxyfuran;2-methyl tetrahydrofuran; dimethylsulfoxide; 1,3-dioxolane; formamide;dimethylformamide; dioxolane; acetonitrile; nitromethane; methylformate; methyl acetate; phosphoric triester; trimethoxy methane;dioxolane derivatives; sulfolane; methyl sulfolane;1,3-dimethyl-2-imidazolidinone; propylene carbonate derivatives;tetrahydrofuran derivatives; ether; methyl propionate; ethyl propionate,etc.

The lithium salt used herein is a substance easily dissolved in thenon-aqueous electrolyte and examples thereof may include LiCl, LiBr,LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆,LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroborane lithium,lower aliphatic carboxylic acid lithium, lithium tetraphenylborate,imides, etc.

Optionally, an organic solid electrolyte or an inorganic solidelectrolyte may be used.

The organic solid electrolyte may include, for example, polyethylenederivatives, polyethylene oxide derivatives, polypropylene oxidederivatives, phosphoric ester polymer, poly agitation lysine, polyestersulfide, polyvinyl alcohol, polyvinylidene fluoride, and a polymerhaving ionic dissociation groups.

The inorganic solid electrolyte may include Li nitrides, halides,sulfates, etc., for example, Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH, LiSiO₄,LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH, Li₃PO₄—Li₂S—SiS₂,and the like.

For improvement of charge-discharge features and/or flame retardancy,the non-aqueous electrolyte may further include, for example, pyridine,triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine,n-glyme, hexaphosphoric triamide, a nitrobenzene derivative, sulfur,quinone imine dye, N-substituted oxazolidinone, N,N-substitutedimidazolidine, ethyleneglycol dialkylether, ammonium salt, pyrrol,2-methoxy ethanol, aluminum trichloride, etc. Optionally, theelectrolyte may include a halogen solvent such as carbon tetrachloride,ethylene trifluoride, etc. to provide non-flammability and/or CO₂ gas toimprove high temperature preservation of the electrolyte.

A lithium secondary battery of the present invention may be fabricatedaccording to any conventional method known in related arts. As to theinventive lithium secondary battery, configurations of the cathode,anode and separator are not particularly restricted and, for example,each sheet may be placed in a circular, angular or pouch type case in awinding or stacking form.

The lithium secondary battery according to the present invention may beemployed in various devices requiring excellent rate properties and hightemperature stability, for example: a power tool driven by an electricmotor; an electric automobile such as an electric vehicle (EV), hybridelectric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), etc.; anelectric two-wheel vehicle such as an E-bike, E-scooter, etc.; anelectric golf cart, and so forth, without particular limitation.

ADVANTAGEOUS EFFECTS

As described above, when a cathode active material of the presentinvention is introduced to a lithium secondary battery, high capacity ofthe battery may be sufficiently attained. In addition, by applying apolymer having a melting point of 80 to 300° C. to a surface of lithiumnickel-cobalt-manganese oxide, high temperature stability may befavorably enhanced without deterioration in performance of the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is an SEM photograph showing a material obtained in Example 1according to Experimental Example 1;

FIG. 2 is an SEM photograph showing a material obtained in ComparativeExample 1 according to Experimental Example 1;

FIG. 3 illustrates a DSC graph of the material obtained in ComparativeExample 1 according to Experimental Example 3;

FIG. 4 illustrates a DSC graph of the material obtained in Example 1according to Experimental Example 3;

FIG. 5 illustrates a DSC graph of a material obtained in Example 2according to Experimental Example 3;

FIG. 6 illustrates an SCC graph of the material obtained in ComparativeExample 1 according to Experimental Example 4;

FIG. 7 illustrates an SCC graph of the material obtained in Example 1according to Experimental Example 4; and

FIG. 8 illustrates an SCC graph of the material obtained in Example 2according to Experimental Example 4.

BEST MODE

Hereinafter, exemplary embodiments of the present invention will bedescribed in greater detail with reference to the following examples.However, those skilled in the art will appreciate that these embodimentsare proposed for illustrative purpose only and do not restrict the scopeof the present invention.

Example 1

After placing an active material: polyethylene in a relative ratio byweight of 100:2 into a dry coating device, the mixture was treated at2.5 kW and 3,000 rpm for 5 minutes. The active material wasLiNi_(0.53)Mn_(0.27)CO_(0.20)O₂.

Example 2

The same procedure as described in Example 1 was repeated to treat theactive material except that a relative weight ratio ofLiNi_(0.53)Mn_(0.27)CO_(0.20)O₂: polyethylene was 100:1.

Example 3

The same procedure as described in Example 1 was repeated to treat theactive material except that a relative weight ratio ofLiNi_(0.5)Mn_(0.3)Co_(0.2)O₂: polyethylene was 100:2.

Comparative Example 1

The active material used in Example 1 was prepared without additionalprocessing.

Experimental Example 1

For each of the active materials obtained in Example 1 and ComparativeExample 1, a surface of the active material was observed through SEM.Observed results of the materials of Example 1 and Comparative Example 1are shown in FIGS. 1 and 2, respectively.

Experimental Example 2

Each of the active materials obtained in Examples 1 to 3 and ComparativeExample 1 was formed into a slurry by blending the active materialtogether with a conductive material and a binder in a relative weightratio of 95:2.5:2.5, followed by applying the slurry to Al-foil so as toform an electrode. After punching the formed electrode to reach 25%porosity, a coin cell was fabricated using the punched electrode. Ananode was Li and an electrolyte was 1M LiPF₆ dissolved in a carbonatesolvent. The fabricated cell was subjected to charge-discharge treatmentat 0.1 C and 3 to 4.25V and, after monitoring capacity and cellefficiency, the results obtained for the materials of Examples 1 and 2and Comparative Example 1 are shown in TABLE 1.

TABLE 1 1^(st) Charge 1^(st) Discharge (mAh/g) (mAh/g) 1^(st) Efficiency(%) Comparative 184.3 162.1 87.9 Example 1 Example 1 183.6 160.0 87.1Example 2 184.1 160.9 87.4

As shown in TABLE 1, it was found that all of the active materialsexhibit favorable electrochemical performance and the active materialobtained in Example 3 shows substantially the same results.

Experimental Example 3

After charging a cell fabricated using each of the active materialsaccording to Experimental Example 2 to 4.3V at 0.1 C, heating positionand heating intensity were measured using a differential scanningcalorimeter (DSC). Results of Comparative Example 1, Example 1 and 2 areshown in FIGS. 3, 4 and 5, respectively.

Compared to Comparative Example 1, it was found that both the activematerials obtained in Examples 1 and 2 have drastically reduced heatingintensity. Also, it was observed that heating position of a main peak,at which the heating intensity was the strongest, shifts toward a highertemperature. In addition, it was observed that the heating intensity isfurther decreased while a position of the main peak shifts toward ahigher temperature when polyethylene content in the active material isincreased. Example 3 also showed substantially the same results asExample 1. Consequently, we determined that safety of the activematerial was preferably enhanced.

Experimental Example 4

For a cell fabricated using each of the active materials according toExperimental Example 2, short circuit current (SCC) was measured. FIGS.6, 7 and 8 show results of Comparative Example 1, Examples 1 and 2,respectively.

Compared to Comparative Example 1, it was found that both the activematerials obtained in Examples 1 and 2 exhibit reduced current peaks.Also, it was observed that, as a content of polyethylene in the activematerial is increased, a size of the current peak is further reduced.Likewise, Example 3 shows substantially the same results as Example 1.Therefore, we determined that safety of the active material waspreferably enhanced.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various applications and modifications are possible onthe basis of the above detailed description, without departing from thescope and spirit of the invention as disclosed in the accompanyingclaims.

1. A cathode active material, comprising: a lithium nickel oxiderepresented by Formula 1, wherein the lithium nickel oxide has a nickelcontent of at least 40% among overall transition metals and is coatedwith a polymer having a melting point of 80 to 300° C.:Li_(x)Ni_(y)M_(1-y)O₂  (1) wherein 0.95≦x≦1.15, 0.4≦y≦0.9, and M is atleast one selected from a group consisting of stable elements atsix-coordination such as Mn, Co, Mg, Al, etc.
 2. The cathode activematerial according to claim 1, wherein the lithium nickel oxide is acompound represented by Formula 1 a:Li_(x)Ni_(y)Mn_(c)CO_(d)O₂  (1a) wherein c+d=1−y, provided 0.05≦c≦0.4and 0.1≦d≦0.4.
 3. The cathode active material according to claim 1,wherein the lithium nickel oxide has an average particle diameter D50 of3 to 20 μm.
 4. The cathode active material according to claim 1, whereinthe lithium nickel-manganese-cobalt oxide is in a secondary particleform comprising agglomerated primary particles, and the primaryparticles have an average particle diameter of 0.01 to 8 μm while thesecondary particles have an average particle diameter of 3 to 20 μm. 5.The cathode active material according to claim 1, wherein the polymer isa material inactivated by an electrolyte for lithium secondary batteriesand an organic solvent.
 6. The cathode active material according toclaim 1, wherein the polymer is at least one selected from polyethylene,polypropylene, polybutylene and polystyrene, or a copolymer or blendcomprising two or more thereof.
 7. The cathode active material accordingto claim 1, wherein the polymer is thoroughly or partially applied to asurface of the lithium nickel oxide.
 8. The cathode active materialaccording to claim 1, wherein the polymer is combined with the surfaceof the lithium nickel oxide by physical bonding.
 9. The cathode activematerial according to claim 1, wherein 20 to 80% of an overall surfaceof the lithium nickel oxide is coated with the polymer.
 10. The cathodeactive material according to claim 1, wherein a coating amount rangesfrom 0.5 to 10% by weight relative to a total weight of the activematerial.
 11. The cathode active material according to claim 1, whereina coating thickness of the polymer ranges from 0.01 to 10 μm.
 12. Alithium secondary battery including the cathode active material as setforth in claim
 1. 13. The lithium secondary battery according to claim12, wherein the lithium secondary battery is used as a power supply fora power tool, an electric vehicle, an electric two-wheel vehicle and/oran electric golf cart.